1. Field of the Invention
The present invention relates to filters, duplexers, and communications devices. More particularly, the present invention relates to a filter, a duplexer, and a communications device for use in radio communications or for transmission/reception of electromagnetic waves, for example, in the microwave or millimeter-wave band.
2. Description of the Related Art
A known resonator for use in the microwave or millimeter-wave band is the hairpin resonator disclosed in Japanese Unexamined Patent Application Publication No. 62-193302. The hairpin resonator allows a more compact construction than a resonator incorporating straight transmission lines.
Another known resonator, which also allows a more compact construction, is a spiral resonator disclosed in Japanese Unexamined Patent Application Publication No. 2-96402. The spiral resonator incorporates spiral transmission lines in order to contain longer transmission lines in a limited area, and also incorporates a resonant capacitor in order to allow even smaller overall dimensions.
The above resonators have each been implemented using a single half-wave transmission line. Thus, in the above resonators, electrical energy and magnetic energy accumulate in separate areas on a dielectric substrate thereof. More specifically, electrical energy accumulates in the proximity of the open ends of the half-wave transmission line, whereas the magnetic energy accumulates in the proximity of the center of the half-wave transmission line.
Such resonators having only one microstrip transmission line are subject to degradation of their characteristics due to the edge effect intrinsic in microstrip transmission lines. More specifically, current concentrates at the edges (side edges and top and bottom edges) of the transmission line as viewed in cross section. Use of thicker transmission lines does not eliminate the above problem.
Accordingly, the present invention provides a filter, a duplexer, and a communications device in which power loss due to the edge effect is significantly reduced and in which the coupling structure between a resonator and input/output terminals does not negatively affect the reduction of the edge effect.
To this end, the present invention, in one aspect thereof, provides a filter having a resonator and a coupling pad. The resonator includes a substrate and a transmission line assembly. The transmission line assembly is constituted of a plurality of spiral transmission lines disposed around a particular point on the substrate so as not to cross one another. The inner ends and outer ends of the plurality of spiral transmission lines substantially define, respectively, an inner circumference and an outer circumference of the transmission line assembly. The coupling pad is disposed at the center of the transmission line assembly, and is capacitively coupled to each of the plurality of spiral transmission lines.
The present invention, in another aspect thereof, provides a filter having a resonator and a coupling pad. The resonator includes a substrate and a transmission line assembly. The transmission line assembly is constituted of a plurality of spiral transmission lines disposed in rotational symmetry with one another around a particular point on the substrate so as not to cross one another. The coupling pad is disposed at the center of the transmission line assembly, and is capacitively coupled to each of the plurality of spiral transmission lines.
The present invention, in still another aspect thereof, provides a filter having a resonator and a coupling pad. The resonator includes a substrate and a transmission line assembly. The transmission line assembly is constituted of a plurality of transmission lines disposed on the substrate. Each of the plurality of transmission lines is represented by either a monotonically-increasing line or a monotonically-decreasing line on coordinates defined by an angle axis and a radius vector axis (see FIG. 2B for example). The line width of each of the plurality of transmission lines does not exceed an angle width of 2xcfx80 radians divided by the number of the transmission lines. The width of the entirety of the transmission line assembly does not exceed the angle width of 2xcfx80 radians at any radius vector. The coupling pad is disposed at the center of the transmission line assembly, and is capacitively coupled to each of the plurality of transmission lines.
In each of the above structures, spiral transmission lines which are substantially congruent with one another are disposed adjacent to one another. Microscopic edge effect appears slightly at the edges of each of the transmission lines; macroscopically, however, the side edges of the transmission lines can be disregarded. Therefore, concentration of current at the edges of the transmission lines is significantly alleviated, reducing power loss. The coupling pad is capacitively coupled to each of the transmission lines by an equal amount of capacitance, so that all the transmission lines have the same resonant frequency so as to achieve a minimized loss.
The coupling pad may be formed on the same plane as the transmission line assembly. This allows fabrication of both the coupling pad and the transmission lines substantially in a single step.
The coupling pad may be disposed so as to partially overlap the transmission line assembly, with a dielectric member interposed between the coupling pad and the transmission line assembly. This provides a greater capacitance between the coupling pad and each of the transmission lines, thereby allowing a smaller coupling pad. Therefore, flexibility of design is enhanced.
The substrate may be laminated onto another substrate provided with an input terminal and an output terminal, the coupling pad being connected, via a bump, to an electrode connected to one of the input terminal and the output terminal. The arrangement serves to allow more compact construction of the filter.
The present invention, in another aspect thereof, provides a duplexer having a filter in accordance with any of the features described above, as one or both of the transmitter filter and the receiver filter in the duplexer. The duplexer is compact and has a low insertion loss.
The present invention, in another aspect thereof, provides a communications device having either a filter in accordance with any of the features described above, or a duplexer as described above. The communications device has a low insertion loss and provides improved communications quality with regard to, for example, noise and transmission rate.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.